1. Field of the Invention
This invention relates to relay driver circuits and, more particularly, to isolating circuits for protecting semiconductor components from large induced voltages during relay switching.
2. Description of the Prior Art
It has become common to operate relays by means of transistor switching circuits. Since semiconductor technology is increasingly taking the integrated circuit form and since the switching of large signals can be best accommodated with relay technology, a transistor driven relay is a convenient interface between these two technologies.
It is likewise known to connect a unidirectional conducting device across a relay coil to prevent large induced reverse voltages from building up across the relay coil when current is interrupted through the cell winding. This arrangement operates well to prevent the driving transistor from being exposed to excessive forward voltages which might cause secondary breakdown. Unfortunately, however, it is desirable to permit a modest reverse voltage during the turn-off portion of a relay operation cycle in order to speed up relay release. A zener diode in series with the unidirectional conducting device permits such a modest reverse voltage. This reverse voltage combines additively with the supply voltage at the switching transistor. Low voltage transistors, particularly when fabricated in integrated circuit form, often break down with the high forward voltages this combination produces.